1. Field of the Invention
The present invention relates generally to materials testing and in particular to improvements in optical rheometric testing.
2. Background of the Invention
Rheometers are widely used to measure viscoelastic properties of fluid materials such as liquids, polymers, and granular solids. As used herein, the term “rheometer” encompasses instruments also referred to as viscometers or viscosometers. A common type of rheometer is a rotational rheometer that typically involves a central shaft attached to a rotating element placed in a sample container. Two distinct types of rotational rheometers are commonly used: a separate motor and transducer (SMT) rheometer and a combined motor and transducer (CMT) rheometer. FIGS. 1a-c illustrate a CMT rheometer 100 having a known arrangement in which the drive system for shaft 102 and measuring transducer 104 are located along a common axis. When a rotation or oscillation is introduced into shaft 102, quartz plate 106 attached thereto rotates in a concentric manner with shaft 102, so that movement or shear can be induced in fluid 108, while lower plate 111 remains fixed. Displacement transducer 104 can be used to determine shear stress introduced in the fluid when a rotational movement is introduced into plate 106. Accordingly, viscoelastic properties of fluid 108 can be studied.
For many applications, in addition to studying viscoelastic response, it is desirable to record fluid optical properties and changes in optical properties in-situ in a rheometer. For example, using a light source, such as stationary laser 110, optical properties can be monitored together with viscoleastic response of fluid 108 when plate 106 is rotated. Sample optical properties such as dichroism, birefringence, and small-angle light scattering can typically be measured using the arrangement of system 100 or similar arrangements. When laser light passes through rotating plate 106 and fluid 108, the optical signal, or change in optical signal can be used to determine certain fluid optical properties such as those listed above. For example, taking into account the optical properties of plate 106, the dichroism or birefringence of fluid 108 can be determined.
Known rheometer systems that include such capability to determine fluid optical properties could in principle be used to study such fluid phenomena such as stress induced birefringence, liquid crystalline transitions, order-disorder transitions, in-situ study of curing reactions, and the like. For example, measured changes in dichroism attributable to fluid 108 can be related to such phenomena as phase separation or phase change in the fluid. However, because of unwanted background interference with an optical signal generated by rheometer components, in-situ measurement of optical properties of a sample is often compromised, as discussed below with further reference to FIGS. 1b and 1c. 
For example, to account for the effect on signal attenuation due to optical plate 106, a background optical measurement of optical plate 106 could be performed in the absence of an experimental sample. Thus, optical plate 106 can be placed into measurement position and exposed to laser 110 so that laser light passes through plate 111 at point C and plate 106 at point A. This provides a “background” optical signal due to attenuation of the laser by the optical plate 106, which can be recorded by detector 114. Referring now to FIG. 1c, an “experimental” optical measurement can subsequently be recorded at detector 114 with liquid sample 108 present, and the background subtracted from the experimental measurement to provide a more accurate estimate of the liquid sample optical properties. However, during collection of a background measurement, the point A used for background measurement may not be representative of optical plate 106, as described further below.
In the parallel plate rheometer geometry illustrated in FIG. 1a, during an actual rheometric experiment, plate 106 is subject to rotation as shown. Thus, different portions of plate 106 are presented to the laser light of the optical probe as the light passes through the plate. Because plate 106 typically exhibits some degree of non-uniformity, as plate 106 rotates, the optical properties of portions of plate 106 that intercept laser light from laser 110 also vary during rotation. This is because the plate may have surface defects, non-uniform thickness, and other defects that vary according to position on the plate. For example, as plate 106 rotates, moving points A and B on optical plate 106 that are exposed to laser 110 at fixed point C, may have different optical properties. In general, the attenuation of laser light passing through plate 106 at point C can vary as plate 106 rotates, causing variation in the optical signal measured after light from laser 110 passes through fluid 108. Thus, the attenuation of laser 110 during a background measurement taken, for example, at point A on optical plate 106, or at any specific point along optical plate 106 between points A and B, might be much different than the attenuation of light on optical plate 106 during measurement of an actual fluid sample, when the laser light beam may be intercepted by many portions of the optical plate 106. The variation in background attenuation of a transmitted signal due to optical plate non-uniformity may overwhelm or substantially affect dichroism or birefringence effects within fluid 108 during a rheometric measurement, by making it difficult to account for the background optical plate contribution to the signal, and therefore making it hard to accurately determine optical properties or changes in optical properties solely attributable to fluid 108.